Introductiontoprogramflowofcontrol,ifand CS112IntroductiontoProgramming
Lecture#7: if-elsestatements
RichardYang
Outline
Admin.andreview Swappingvaluesoftwovariables Conditionalstatements
Introductiontoprogramflowofcontrol
2
Admin.
Assignment1duethisevening11:59pm r Ifyouhaveanytrouble,talktomeaftertheclass r Iwillbeavailableintheafternoonfrom5:006:00pminmy office(AKW202) Assignment2duenextMonday r startearly!!
3
Review:DataTypes
Datatypes
r r r
TwokindsofdatatypesinJava
Primitivedatatypes Userdefinedtypes(orclasses)
wewillstudyclassesindepthafterwehavecoveredprogramstatements
Dataandallowedoperations
Variables
Manyclasses
r r r r
int count; Random myRandom;
// we also say myRandom is an object
String Random KeyboardInput:theKeyboardclass Terminaloutput: Graphics
NumberFormat DecimalFormat
r
manymethodstodrawdifferentshapes
4
Review:TwoTypesofJavaPrograms
Applications r Definethemainmethod Applets
r
Manyspecialmethodsyoucandefine r Sofar,themostimportantoneisthepaintmethod,whichwill becalledbyabrowser r TheinputtopaintisaGraphicsobject
r Appletdemos: Sundemos
http://java.sun.com/j2se/1.4/docs/relnotes/demos.html
Audio http://pantheon.yale.edu/~yry3/audio/audio.html
5
Outline
Admin.andreview Swappingvaluesoftwovariables Conditionalstatements
Introductiontoprogramflowofcontrol
6
SwappingValuesofTwoVariables
Howabout?
x = y; y = x;
Value stored in x y a b b b b b
Usetwotemporaries:
t1 t2 x y
= = = =
x; y; t1; t2;
7
SwappingValuesofTwoVariables
Justonetemporary:
t1 = x; x = y; y = t1;
Notemporaries!
x = x + y; y = x - y; x = x - y;
Dontwritesuchcode!!
Value stored in x y a b a+b b a+b a b a
8
Outline
Admin.andreview Swappingvaluesoftwovariables Conditionalstatements
Introductiontoprogramflowofcontrol
9
FlowofControlofProgramStatements
Theorderofstatementexecutioniscalledtheflowofcontrol
r
Someprogrammingstatementsmodifyflowofcontrol,allowingusto:
r
Withoutthese,theorderofstatementexecutionthroughamethodislinear:oneafter
decidewhetherornottoexecuteaparticularstatement;thesearealsocalledthedecision statements,orconditionalstatements performastatementoverandoverrepetitively;thesearealsocalledrepetitionstatements,or loopstatements
theotherintheordertheyarewritten Withdecisionmakingandrepetitionstatements,youcanwritesubstantialprograms
10
Outline
Admin.andreview Swappingvaluesoftwovariables Conditionalstatements
Introductiontoprogramflowofcontrol
11
ConditionalStatements
Aconditionalstatementletsuschoosewhichstatement
willbeexecutednext Conditionalstatementsgiveusthepowertomakebasic decisions Java'sconditionalstatements:
theif statement r theif-elsestatement r theswitchstatement
r
12
Outline
Admin.andreview Swappingvaluesoftwovariables Conditionalstatements
Introductiontoprogramflowofcontrol
ifandif-else statements
statementstructure specifyingconditions:booleanexpressions
13
TheifStatement
Theifstatementhasthefollowingsyntax:
if is a Java reserved word The condition must be a boolean expression. The boolean e.g., a boolean variable, a == b, a <= b. e.g., It must evaluate to either true or false. It
if ( condition ) statement;
If the condition is true, the statement is executed. If it is false, the statement is skipped.
14
TheifStatement:Examples
Anexample:
if (sum > MAX) delta = sum - MAX; System.out.println ("The sum is " + sum); First, the condition is evaluated. The value of sum First, sum is either greater than the value of MAX, or it is not. is MAX If the condition is true, the assignment statement is executed. If it is not, the assignment statement is skipped. Either way, the call to println is executed next.
SeeAge.java(page135)
15
Logicofanifstatement
condition evaluated true statement false
16
Theif-elseStatement
Anelseclausecanbeaddedtoanifstatementtomakeitan
if-elsestatement:
Iftheconditionistrue,statement1isexecuted;ifthe
if ( condition ) statement1; else statement2;
conditionisfalse,statement2isexecuted
Oneortheotherwillbeexecuted,butnotboth SeeWages.java(page139)
17
Logicofanif-elsestatement
condition evaluated true statement1 false statement2
18
WhatifIWanttoRunSeveralStatements:Block Statements
Severalstatementscanbegroupedtogetherintoablock
statement Ablockisdelimitedbybraces({}) Ablockstatementcanbeusedwhereverastatementis calledforintheJavasyntax Forexample,inanif-elsestatement,theif portion,ortheelseportion,orboth,couldbeblock statements SeeGuessing.java(page141)
19
NestedifStatements
Thestatementexecutedasaresultofanifstatementorelse
clausecouldbeanotherifstatement Thesearecallednestedifstatements
SeeMinOfThree.java(page143)
20
Unbalancedif-elseStatements
Example:
if (code == R) if (height <= 20) System.out.println(Situation normal!); else System.out.println(Bravo!);
Considerthreescenarios r code=R;height=20; r Code=R;height=21; r Code=T;height=20; r Code=T;height=21;
Rule:Anelseclauseismatchedtothelastunmatchedif(nomatterwhat
theindentationimplies)
21
Outline
Admin.andreview Swappingvaluesoftwovariables Conditionalstatements
Introductiontoprogramflowofcontrol
ifandif-elsestatements
statementstructure specifyingconditions:booleanexpressions
22
BooleanExpressions:Basics
AconditionoftenusesoneofJava'sequalityoperators(==,!=)or
relationaloperators(<,>,<=,>=),whichallreturnbooleanresults:
== != < > <= >=
Notethedifferencebetweentheequalityoperator(==)andthe
equalto notequalto lessthan greaterthan lessthanorequalto greaterthanorequalto
assignmentoperator(=)
23
ComparingCharacters
TheresultsarebasedontheUnicodecharacterset Wecanalsousetherelationaloperatorsoncharacterdata Thefollowingconditionistruebecausethecharacter'+'comesbefore
thecharacter'J'inUnicode:
if ('+' < 'J') System.out.println ("+ is less than J");
Theuppercasealphabet(AZ)andthelowercasealphabet(az)both
appearinalphabeticalorderinUnicode
24
ComparingStrings
RememberthatacharacterstringinJavaisanobject Wecannotusetherelationaloperatorstocomparestrings
Theequalsmethodcanbecalledonastringtodetermineiftwo
stringscontainexactlythesamecharactersinthesameorder TheStringclassalsocontainsamethodcalledcompareToto determineifonestringcomesbeforeanotheralphabetically(as determinedbytheUnicodecharacterset)
25
ComparingFloatingPointValues
Becarefulwhencomparingtwofloatingpointvalues(floator
double)forequality Youshouldrarelyusetheequalityoperator(==)whencomparingtwo floats Inmanysituations,youmightconsidertwofloatingpointnumberstobe "closeenough"eveniftheyaren'texactlyequal Therefore,todeterminetheequalityoftwofloats,youmaywanttouse thefollowingtechnique:
if (Math.abs (f1 - f2) < 0.00001) System.out.println ("Essentially equal.");
26
MoreComplex(Compound)Boolean Expressions:LogicalOperators
Booleanexpressionscanalsousethefollowinglogical
! LogicalNOT && LogicalAND || LogicalOR Theyalltakebooleanoperandsandproduceboolean results LogicalNOTisaunaryoperator(ithasoneoperand),but logicalANDandlogicalORarebinaryoperators(they eachhavetwooperands)
27
operators:
LogicalNOT
ThelogicalNOToperationisalsocalledlogicalnegation
orlogicalcomplement Ifbooleanconditionaistrue,then!aisfalse;ifais false,then!aistrue Logicalexpressionscanbeshownusingtruthtables
a true false
!a false true
28
LogicalANDandLogicalOR
Thelogicalandexpressiona && bistrueifbothaandbaretrue,andfalse
otherwise
r
Thelogicalorexpressiona || bistrueifaorborbotharetrue,andfalse
shortcut:ifaisfalse,bisnotevaluated,e.g., if (count != 0 && total /count) { } shortcut:ifaistrue,bisnotevaluated
otherwise
r
a true true false false
b true false true false
a && b true false false false
a || b true true true false
29
PrecedenceofBooleanOperators
Equalityoperators(==,!=)andrelationaloperatorshave
thehighestpriority NOT(!)ishigherthanAND(&&),whichishigherthan OR(||) Ifequalprecedence,fromlefttoright
Example:BooleanOrder.java
30
BackupSlides
Outline
Admin.andreview Swappingvaluesoftwovariables Conditionalstatements
Introductiontoprogramflowofcontrol
ifandif-elsestatements
Javaconditionaloperator switchstatement
statementstructure specifyingconditions:booleanexpressions
32
TheConditionalOperator
Javahasaconditionaloperatorthatevaluatesabooleancondition
thatdetermineswhichoftwootherexpressionsisevaluated Theresultofthechosenexpressionistheresultoftheentire conditionaloperator Itssyntaxis:
condition ? expression1 : expression2
Iftheconditionistrue,expression1isevaluated;ifitisfalse,
expression2isevaluated
33
TheConditionalOperator
Theconditionaloperatorissimilartoanif-else
statement,exceptthatitisanexpressionthatreturnsa value Forexample:
larger = (num1 > num2) ? num1 : num2;
Ifnum1isgreaterthatnum2,thennum1isassigned
to...